The present invention relates to a composite head assembly in a disk drive. In particular, the present invention relates to a head assembly which incorporates glass as a bonding material between the core and the slider on both the side of the core proximate the slider rail surfaces and on the side of the core opposite the rail side.
In a magnetic disk drive, information is stored in the form of magnetically polarized bit positions on a rapidly rotating disk. Information bit positions are arranged in generally concentric data tracks on the disk surface, and the tracks are further subdivided into sectors. Information transfer to and from the disk is accomplished by a transducer mounted in an air-bearing slider.
The transducer and air-bearing slider together comprise a head assembly. The slider is typically a catamaran-type, having two parallel "rail" surfaces which face the surface of the magnetic disk. These two surfaces are appropriately called the air bearing surface (ABS). A head suspension assembly positions the head assembly and applies a spring preload force, forcing the head assembly toward the disk's surface. Viscous drag forces cause a thin layer of air molecules to adhere to the surface of the disk when it is rotated. This layer of air is pulled under the slider's ABS and causes the head assembly to lift off the disk surface when sufficient rotational speed is reached. Thus, the head assembly flies in close proximity (approximately 10 microinches) to the disk surface, precisely balanced between aerodynamic forces and the applied preload spring force.
In a composite head assembly, a transducer is mounted in a ceramic slider. The transducer is comprised of a magnetic core (usually ferrite) and a coil of electrically conducting bifilar wire. The core is comprised of two sections. The first section of the core is a straight rectangular piece called the "I"-section. The second section of the core resembles a squared off C-shape and is called, appropriately enough, the "C"-section. When the two sections are joined during manufacturing, the completed core is essentially rectangular in shape and has a large opening in the center. The bifilar wire is wrapped (typically 30-60 turns) through the central opening and around a portion of the I-section. The upper edge, perpendicular to the segment of the I-section about which the wire is wrapped, is called the "track."
The track side of the core is machined down to a narrow thickness. The track side faces the magnetic disk in the final assembly and is positioned such that the track side is flush with the slider's rail surface and parallel to the information tracks on the disk.
A small space between the I-section and the C-section in the track side of the core defines the gap. The gap causes a disturbance in the magnetic flow through the core. This disturbance causes flux leakage necessary for the completed transducer to couple magnetically with the disk. The dimensions of the gap are critical to the electromagnetic characteristics of the transducer and proper functioning of the head assembly. The gap width is determined by the thickness of the track side of the core. The gap width is a limiting factor in the narrowness of the data tracks obtainable on the disk and, thus, directly influences the amount of information that can be stored on the disk.
The gap length is the distance between the edges of the I-section and the C-section where the two join on the track side of the core. The gap length is controlled by the thickness of a sputtered film placed on adjoining surfaces of the I-section and the C-section prior to their joining. The sputtered film also serves to act as a physical bonding agent between the I-section and the C-section. The gap length directly influences the minimum size of the bit positions on the data track and the amount of information that can be stored on the disk.
The upper segment of the C-section, where it forms the portion of the core's track side adjacent the gap, is narrowed by means of an angle formed in the inner surface which defines the C-section's portion of the core's central opening. This narrowing to a squared-off point is necessary to create the necessary flux leakage at the gap. The thickness of this squared-off point measured parallel to the I-section-to-C-section joining line is referred to as throat height. Accurate control of the throat height dimension during manufacturing is critical to predictable electromagnetic performance of the transducer.
The "back leg" of the core is the side of the C-section opposite and parallel the track side.
Prior art composite head manufacturing methods involve first inserting the core into machined upper and lower slots in the air-bearing slider and temporarily attaching the core to the slider with a small quantity of adhesive. A measured quantity of sealing glass is then laid on top of the track side of the core. Next, the slider and head are heated in an oven until the sealing glass melts and flows down into the upper slot between the core and the slider. The slider and core assembly is then cooled until the sealing glass resolidifies, forming a bond. Next, a liquid epoxy is introduced into the lower slot between the core and the slider.
The assembly is then heated until catalytic reactions in the epoxy cause it to solidify. After hardening of the epoxy, the slider's air bearing surfaces are lapped to a final finish dimension. The lapping process removes excess sealing glass from the slider surface and brings the track side of the core flush with the air bearing surface. Finally, bifilar wire is wrapped around the I-section in the area between the slots of the slider. The winding is done through a central opening in the core and through a winding window area which has been formed in the slider prior to insertion of the core into the slots.
Some problems inherent in head assemblies manufactured in accordance with the prior art are directly attributable to the use of epoxy in bonding the core's back leg side to the slider.
The presence of the machined slots and a winding window in the slider reduces its structural integrity. Epoxy exhibits more flexibility and a tendency to deform under load than the ceramic of which the slider is comprised. The epoxy's lack of rigidity reduces the ability of the core to restore the structural integrity of the side of the slider in which it is mounted. During final lapping of the slider rail surfaces, the portion of the slider in which the core is mounted deforms due to flexing of the epoxy bond. This deformation results in lack of straightness of the slider rail surface in the core area and impacts detrimentally on the head assembly's flying characteristics.
Another problem with prior art epoxy bonding of the back leg is flexing of the rail directly under the I-section during wire winding, resulting in rail deformation and protruding. This projection is called a "tail dragger" and its presence can directly effect fly characteristics. Improper fly height can adversely impact the electromagnetic performance of the transducer.
Finally, prior epoxy bonding is a slow manual method that can result in additional labor costs. Epoxy in liquid form is difficult to handle and changes viscosity over time. If the viscosity is too low when the epoxy is applied, it will flow too easily into places such as the winding area of the I-section where it must be removed before further processing. Epoxy present on the winding area can result in costly waste if unnoticed and the manufacturing process is allowed to proceed. If the viscosity of the epoxy is too high when it is applied, it will not flow well into the cavity between the slots in the slider and the core. If insufficient epoxy is present, a poor bond is achieved and more deformation of the slider during lapping and winding will be evident.